1. Field of the Invention
The present disclosure relates generally to methods and systems for performing acoustic measurements of subterranean formations. More specifically, some aspects disclosed herein are directed to methods and systems for deploying seismic devices, such as seismic sources and/or receivers, in a borehole for characterizing subterranean formations having, for example, oil and/or gas deposits therein. The methods and systems utilize permanent downhole installations that removably retain retrievable seismic tools having one or more seismic instrumentation such that the seismic instrumentation has a fixed location and orientation relative to the subterranean formations.
2. Background of Related Art
Seismic exploration can provide valuable information useful in the drilling and operation of oil and gas wells. Seismic measurements of the type described herein are also useful in the fields of CO2 sequestration, development of methane hydrate deposits, water reservoir monitoring, microearthquake monitoring, and monitoring for reservoir delineation, among other applications that are known to persons skilled in the art. Seismic measurements are conducted with energy that is introduced by a seismic source to create a seismic signal that propagates throughout the subterranean formation. This seismic signal is reflected to differing degrees by features that are of interest. A receiver monitors these reflected signals to help generate a seismic map of the underground features. This map is generated by knowing the exact time that a seismic signal was generated as compared to the time that the reflected signal is received. As a practical matter, the system comprises a plurality of sources and receivers to provide the most comprehensive map possible of subterranean features. Different configurations may yield two dimensional or three dimensional results depending on their mode of operation.
In typical seismic exploration, it is necessary to cover a large surface area where the deposits of interest, such as oil and/or gas, are located. Since productivity is key for this business, the seismic devices, such as the source(s) and receiver(s) and associated electronics, are moved to different locations to cover the area of interest. In this, such seismic surveying operations require efficient and fast acquisition of seismic measurements.
Long term monitoring in contrast is geared towards the periodic monitoring of seismic events and/or the slow movement of fluids in a subterranean reservoir. A reservoir extends over a limited area. Typically the length of data acquisition time is less of a constraint. In this, small changes in the reservoir are detected using, for example, time lapse techniques. Seismic data acquisition is governed by the timing of the seismic events or changes in the reservoir. Small changes in seismic wave reflection are detected by locating the seismic devices, such as the source and receiver, at the same position with the same orientation relative to the subterranean formations, and the same coupling condition. However, an instrument has limited life and eventually it fails. If the instrument is retrieved from its deployed position, for example, for maintenance, it is not possible to easily re-deploy the instrument at the same location, in the same orientation and with the same coupling condition. This poses a problem in long term monitoring operations.
In a typical seismic survey operation utilizing, for example, vibroseis sources, large vibrator trucks are deployed to introduce seismic energy into the subsurface for purposes of reservoir imaging. Such systems are very large in size and costly. Impulsive sources such as an airgun or dynamite provide a large amount of seismic energy instantaneously for a relatively short duration of time, typically in the order of a few hundred milliseconds. In contrast, a vibrator provides low seismic power, but sweep time is comparatively long, typically 20 to 30 seconds. Hence, the total amount of energy that is generated is the same for the two types of source devices.
As mentioned above, typically fast data acquisition is not a factor in relation to permanent sources with fixed receivers that are used in, for example, reservoir monitoring. Data acquisition may extend for one hour or more. Therefore, it is practical to deploy a small seismic source downhole for purposes of seismic surveying by using the source for a suitable length of time to generate the appropriate amount of seismic energy.
Table 1 below shows an order of magnitude comparison of the different types of seismic sources discussed in the preceding.
P [W]t [sec]E [J]Airgun10000.2200Vibrator1020200Permanent source0.13600360
In view of the foregoing, the applicants recognized that it would be advantageous to deploy downhole seismic devices in a manner that is most cost efficient and reliable for purposes of long term seismic monitoring. Applicants discovered that it is possible to utilize a compact seismic source system that is deployable in a borehole for purposes of seismic surveying. However, seismic sources need comprehensive maintenance and support. Therefore, conventional seismic sources are often not suitable for permanent or semi-permanent deployment to monitor subsurface reservoirs.
Alternative seismic energy sources such as airguns and dynamite also are known in the art. However, in certain applications, such as active permanent monitoring, the aforementioned seismic energy sources have shortcomings such as safety, size, cost, required maintenance, repeatability, and environmental impact.
The applicants further recognized that it is advantageous to have retrievable seismic devices, such as seismic sources and receivers, for downhole use. Available downhole seismic instrumentation tends to be fragile and may not last for an extended period of time without periodic maintenance. Downhole seismic instruments are expensive, and it is wasteful to permanently deploy them in a borehole if their use is limited. It is preferable to retrieve the seismic devices after a seismic surveying operation and to redeploy the devices when needed for seismic surveying. In this way, the same seismic devices may be deployed at different wellsite so that overall costs and depreciation are reduced. However, when redeployed at a previous wellsite it is necessary that the seismic devices be located at the same position, in the same orientation, and with the same coupling condition as the previous seismic operation.
There is need for improved methods and systems for deploying seismic devices for purposes of acoustically monitoring subterranean formations to derive key parameters relating to the formations. Specifically, there is need for techniques for deployment of seismic sources and receivers in a safe manner with low environmental impact for purposes such as active or passive monitoring with high repeatability. For example, it is desirable to deploy seismic devices at transition zones, such as at a swamp or shallow water lake/sea, by anchoring or latching the devices in the hole.
In this, one object of the present disclosure is to provide an improved mechanism for deployment of downhole seismic sources and receivers. Another object of the present disclosure is to enable deployment of seismic devices by anchoring in a well or a hole for seismic acquisition.